Granulates can be produced from powder-form solids by various processes, for example by pressing, extrusion, rolling granulation and drop granulation (cf. for example H. B. Ries, Aufbereit. Techn. 3/5, page 147, 1970).
The last two methods give spherical granulates which may be used with advantage for adsorption purposes by virtue of their favorable flow properties. Solids which do not have any binding power of their own require a binder for conversion into a granulate of adequate strength. Silica sol/silica gel and silicate solutions have already been used for that purpose.
Silica-bound granulates may be produced using silicate solutions and acid or stable silica sol. Sols of silicate solution and acid have the disadvantage that their scope for processing is too narrow for application on an industrial scale.
In addition, some materials which it is desired to convert into granulate form are unable to tolerate acidic reactions. This is the case, for example, with most of the molecular sieve zeolites used for adsorption purposes.
Several processes based on the sol-gel method using the solidification of an unstable sol of silicate solution and acid, with which finely powdered solids may also be mixed, in inert solvents have already been proposed for the production of catalysts (cf. for example U.S. Pat. No. 2,689,226, German Offenlegeschrift 24 13 284).
Molecular sieve zeolites may be similarly converted into spherical granulates by the sol-gel method using stable silica sol and adding a small amount of a magnesium oxide suspension as gelating agent. Although processes such as these produce a very uniform spherical granulate, they are relatively complicated and, on account of the water-immiscible solvent, require elaborate measures for removing and working up the solvent.
It has also been proposed (German Offenlegeschrift 22 33 070) to granulate molecular sieve zeolites by rolling granulation on a granulating pan in which originally stable silica sol adjusted shortly before use to a pH-value of 4-9 by the addition of a small quantity of acid to reduce stability is sprayed onto the zeolites. In this process, the solgel conversion takes place during rolling granulation in the pan. Granulates with very weak SiO.sub.2 -bridges between the zeolite particles are formed in this process. In addition, the formation of salts with the necessary alkaline reaction media during subsequent conversion of the SiO.sub.2 -binder into zeolite is a disadvantage.
German Auslegeschrift 11 64 995 describes the production of molecular sieve granulate by mixing the hydrated washed zeolite with stable silica sol which normally has a pH-value of from 9 to 10.
The solidification of the silica-sol-containing zeolite granulate, i.e. the conversion of the sol into the gel, only takes place at temperatures above 100.degree. C. or during the subsequent activation step at 300.degree.-350.degree. C. One of the disadvantages of this process is that the granulates are only capable of withstanding very light mechanical loads before the sol-gel conversion and, accordingly, have to be treated very carefully in order not to damage the structure of the granules, for example during sifting or transport.
In the above-mentioned sol-gel processes, the granules harden suddenly after mixing following an induction period of a few seconds. A hardening process which begins suddenly after only a few seconds is hardly suitable in cases where granulation is carried out by pressing, extrusion, roll granulation or by the preferred process of build-up granulation into beads.
These disadvantages are avoided by a process known from German Offenlegeschrift 26 42 518 in which small quantities of finely powdered alkali silicate are dry-mixed with the solid to be granulated so that a certain molar ratio of Me.sub.2 O to SiO.sub.2 is adjusted during addition of the silica sol and the granulate is subjected to ageing treatment. The advantage of this process lies in a length of time favorable for processing on an industrial scale. Hardening begins gradually rather than suddenly after an induction period of a few minutes to a few hours which may be regulated through the molar ratio of Me.sub.2 O to SiO.sub.2.
The disadvantage of this known process is that very small quantities of alkali silicate have to be mixed with a large quantity of zeolite. The difficulty here is that small quantities of solids have to be measured off within the necessary tolerance using either mechanical balances or similar metering systems in a continuously operating installation. Another problem is obtaining adequate distribution of the alkali silicate in the large quantity of zeolite in a reasonable mixing time.